A standby or uninterruptible power supply, referred to herein generally as backup power systems, provide output power to a load when the main input power, usually from commercial power lines, fails. Typical backup power systems have an inverter to convert the DC voltage provided from a storage battery to the required AC output voltage. A static switch or relay may be utilized to isolate the AC output voltage from the failed input power. A common prior art arrangement for a backup power system is illustrated in the simplified schematic of FIG. 1. AC input power from a main source, such as a commercial power system, is received on input lines 10 and 11, and output power is delivered on output lines 12 and 13 to the critical load (not shown). A static switch 14 is connected in a line 15 between the input line 10 and the output line 12 to isolate the input from the output upon failure of the main AC power system, while a line 16 directly connects the input line 11 and the output line 13. Upon failure of the main AC power, a controller (not shown) detects the failure of the AC line power, opens the static switch 14, and turns on an inverter 17 to convert DC voltage, from a storage battery 18, to AC voltage at the inverter output terminals which is provided through a power transformer 19 across the AC output lines 12 and 13. During normal operation when the inverter 17 is off, a battery charger 21 provides DC charging current to the battery 18 by rectifying AC power from a charger transformer 23 which is connected by lines 24 and 25 to the AC power lines. The charger transformer 23 and the power transformer 19 serve in large part to isolate the typically low voltage DC circuits of the battery charger and inverter from the high voltage AC line circuits.
A problem commonly associated with a backup power system of the type shown in FIG. 1 is that while the AC input voltage is present, the inverter 17 is turned off. It is desirable to have a way of testing the inverter to verify that the inverter is in good operating condition, since the reliability of the inverter is crucial to the backup power system's mission. In the prior art, such inverter testing requires additional circuitry and controls, and thus added expense.
The battery chargers used in various backup power systems vary widely in circuit topology, but have common problems. The most significant problem arises from the rectifier/capacitor filter combination typically used to convert the AC voltage from the charger transformer to the DC voltage required by the battery, because current is drawn in bursts or pulses at the crest of the input voltage waveform. This causes harmonic distortion of the AC input current. Phase-controlled battery chargers cause even worse distortions. While solutions exist to solve this problem, such solutions typically require additional circuitry and thus expense. The charger circuitry also adds expense to such backup power systems, with the cost of the chargers being generally proportional to charging current levels.